It is widely recognized that the conventionally used kinds of energy sources available to the U.S. as well as to the rest of the world are strictly limited and that many presently used techniques of energy conversion have highly undesirable side effects. No documentation need be cited to support the assertion that available fossil fuel materials are finite in quantity, even though estimates of remaining quantities differ. Oil and gas may be shortest in supply and even domestically available uranium ore resources are limited. While coal quantities are greater in terms of years of supply remaining, these are also clearly finite and combustion of coal of low grade (high sulfur) is a serious pollutant requiring extensive and expensive effluent cleaning operations. Thus, considerable attention has been given to improving known sources of energy and to developing new sources, particularly of electrical energy. Various alternatives have been considered.
Among these alternatives are solar energy, which has generally been regarded as either not usable for electric power generation or usable only in the event of the development of new conversion techniques. In an effort to develop new techniques, some research efforts are in progress and background information on certain aspects of this research are described at length in a document, "Energy Conversion Alternatives Study," NASA-CR 134948 (1976), prepared for the National Aeronautics and Space Administration and ERDA under a government contract.
One solar conversion technique which is being investigated uses an apparatus which includes an array of solar reflectors which concentrate solar radiation on a tower. The concentrated solar energy is used to heat water in the tower and the heated water vapor, in the form of steam, drives turbines, the turbines being used to mechanically drive rotary generators for producing electricity. While this appears to be an interesting and promising approach, and has the considerable advantage of avoiding burning fossil fuels, it is not a direct conversion system and still relies, as have earlier solar generating systems, on elevating the temperature of water or some other working fluid which is then used to mechanically, or thermodynamically, drive rotary generators. The capital investment in such a system is therefore necessarily quite large because it is ultimately restricted by steam turbo-generator efficiencies limited to about 40%. The systems as proposed would appear to be usable only as a "peak load" generator during daylight hours.
Indeed, solar energy generation systems have generally been viewed only as "peak load" generating systems, and it has been thought that solar energy would not present a viable "base load" generating approach, at least until further technological development has been accomplished, for two primary reasons, namely, that any specific location on the earth's surface, solar radiation is available for only about one-half of each day, more or less; and that batteries or some other means for storing electrical energy produced during those daylight hours for use during hours of darkness are expensive, relatively inefficient, and are limited in their power handling capabilities. It is in the area of storage technology that further development has been deemed essential.
Another alternative which has been and is being extensively investigated is the use of magnetohydrodynamic (MHD) generators. The patent and other literature on MHD systems is voluminous and the variety of system approaches is too great to permit even a cursory discussion of them in the present context. It is well known that such systems involve the production of a plasma which is caused to pass between electrodes and through a magnetic field. The plasma is a high temperature gas, fully or partially ionized, so that it contains ions, electrons and, if not fully ionized, neutral particles. The dynamic behavior of the charged particles responds to the presence of electric and magnetic fields. Thus, when a moving "stream" of plasma passes through a magnetic field normal (orthogonal) to the flow direction of the stream, oppositely charged particles are subject to opposing forces in the third orthogonal direction. Appropriately placed electrodes and circuit means can then be used to collect the charged particles as electrical current.
In known MHD power generating systems, the plasma itself is most commonly formed from combustion products of a fossil fuel mixed with compressed air. The largely gaseous product of the combustion is partially thermally ionized as a result of the combustion and the ionization is enhanced in recent technology by the addition of a "seeding" element such as an alkali metal such as cesium. A discussion of some of the theoretical aspects of MHD generators can be found in several articles in the IEEE Proceedings of September, 1968, (Vol. 56, No. 9), which was a special issue devoted to MHD.
Systems of this type have the significant advantage of providing efficient use of fossil fuels in direct power generation, but have the disadvantage of still using fossil fuel as the primary energy source. In addition, there are problems associated with the existence of fly ash and other impurities in the gas which forms the plasma in that these impurities tend to corrode and accumulate on the interior components of the MHD system itself, requiring provision for cleaning with the attendant necessity for shutting down the system.
The foregoing MHD systems are sometimes referrred to as "open cycle" systems in that the gas forming the plasma is discharged from the system after passage through the MHD generator. As will be recognized, the effluent gas must be "cleaned" before discharge to remove pollutants, much as with any other fossil fuel burning system. Also, the high temperatures at which such systems operate create difficult materials problems which have yet to be solved.
Another form of MHD system is referred to as being "closed cycle" because the plasma is retained within the system and is recycled for reuse. While a closed cycle system would have obvious advantages, including the absence of polluting effluents and efficiency at the stack, no practical power generating system based on this principle is known in the prior art.